We examine the problem of evaluating the performance of TCP connections over wide area networks. Our approach combines experimental and analytic methods, and proceeds in three steps. First, we have used measurements taken over Renater to provide a basis for the chosen analytic model. This model turns out to be a shared bottleneck model, in which a finite buffer queue is shared by two connections, one being the reference TCP connection, and the other representing the exogenous traffic (i.e. all the other connections). This model is unlike previous models which did not explicitly consider the impact of exogenous traffic on the reference TCP connection. Second, we use fluid modeling to analyze the behavior of the reference TCP connection. We identify two modes of operation. For each mode, we derive closed-form expressions for the average throughput and delay as a function of buffering, roundtrip delay, and characteristics of exogenous traffic. Third, we use simulation to validate the model, and we find in general good correlation with the analytic results.